The present invention relates to an apparatus for supporting a glass sheet during a tempering process, and more particularly, to an apparatus which supports and cushions the glass sheet during a tempering process without adversely affecting the optical qualities of the resulting tempered glass.
Various apparatus and devices for supporting hot glass sheets during a tempering process are known in the art. One such device comprises a ring-type mold made of rigid metal that includes an outline upper surface shaped to conform to the shape desired slightly inward of the outline of the glass sheet. Typically, a glass sheet is heated in a furnace to its softening point, and then either pressed to shape between complementary molds and transferred to the ring-type mold, or sagged by heat and gravity to the shape provided by the shaping surface of the mold.
After the glass sheet is shaped, it is transferred while supported on the ring to a quenching station where it is quenched rapidly by applying cold air blasts against the upper and lower surfaces of the glass sheet. Such a tempering process greatly increases the mechanical strength of the glass and its resistance to breakage. Further, when fractured, tempered glass tends to break into relatively small, smooth pieces rather than large, jagged fragments.
However, the ring-type mold that supports the glass sheet as it is transferred between the shaping station and the quenching station is alternately exposed to the hot temperature of the furnace and to the lower temperature provided by cold blasts of air at the quenching station. When a shaped glass sheet contacts the hot, heavy metal ring during this rapid cooling, glass breakage can result at the glass-metal contact areas due to the difference in heat conductivity between the glass sheet and the metal ring.
Further, when the heat-softened glass comes into direct contact with the rigid ring, slight blemishes or variations from the shape desired become obvious on the surface of the shaped glass, These blemishes disrupt the optical properties of the shaped glass.
Accordingly, there have been attempts to eliminate these problems by avoiding direct contact of the glass sheet with the metal ring, thus preventing excessive heat transfer which may cause cracking. For example, McMaster, U.S. Pat. No. 3,586,492, and Seymour, U.S. Pat. No. 3,741,743 disclose an endless ring frame for supporting a glass sheet having a plurality of teeth around the frame which are covered with a wire mesh screen to minimize contact of the ring with the glass sheet. In Seymour the wire mesh screen includes a heavy wire mesh supported on the ring and a fine wire mesh that contacts the glass sheet. In other applications, a wire mesh cloth has been used to cushion the glass and minimize contact with the ring. However, in order to provide the desired cushion for the glass sheet, many layers of mesh are required, which not only adds expense but also decreases the quench air flow to the glass. Further, mesh cloth is typically not very durable, and must be replaced frequently.
Yamada et al, U.S. Pat. No. 4,749,399 describes an apparatus for shaping a curved tempered glass sheet including a cooling ring mold covered with a punched metal strip having a number of openings to improve quench air flow and to prevent the cooling ring from coming into direct contact with the glass. However, such a metal strip is often difficult to fit on frames.
Accordingly, there is still a need in the art for an apparatus for supporting a glass sheet during a tempering process which provides increased cushioning of the glass without restricting quench air flow, and without adversely affecting the optical quality of the glass.